Add support for 'is.uri'

[joeltankam]

Checks if the given value is an uri.

is.uri('http://www.test.com'); // true
is.uri('ftp://www.test.com'); // true
is.uri('https://www.test.com:8081?foo=bar'); // true
is.uri('test'); // false

Checkout full uri description on wikipedia.

Update README.md: under is > RegExp

[beast911]

i would like to help with this

[joeltankam]

Feel free to contribute

[beast911]

I have raised a PR for this. Kindly check